Method for controlling bowing in light valves

ABSTRACT

A method is disclosed for controlling the bowing of the walls of light valves. This is especially necessary when large sized valves contain a liquid suspension. To reduce this distortion of shape and thereby eliminate bowing, an amount of fluid is withdrawn from the cell prior to sealing the cell hermetically. Fluid is withdrawn, thereby reducing the pressure on the inside of the walls, until the pressure at least one point inside the walls, and preferably toward the lower center of a light valve positioned vertically, is substantially equal to the atmospheric pressure on the outside of the light valve walls.

1 United States Patent 11 3,7443% Forlini et al. July 10, R973 [54]METHOD FOR CONTROLLING BOWING IN 3,655,267 4/1972 Forlini 350/150 LIGHTVALVES FOREIGN PATENTS OR APPLICATIONS ll'lVl'ltOlSZ Matthew Fflllllll,OZOHO Park; 4 7 032 93 Great Britain Frailcis Cr Lowell, Huntington1,108,593 4/1968 Great Britain Station, both of NY. 1,138,992 1/1969Great Britain [73] Assignee: Research Frontiers Incorporated,

pl i i N Y Primary Examiner-Charles W. Lanham Assistant Examiner--V. A.DiPalma [22] FIed: July 1971 Attorney-Stephen F. Feldman [21] Appl. N0.:166,014

[57] ABSTRACT [52] HS. Cl 29/592, 29/421, 161/45, A method is disclosedfor controlling tha bowing ofthc 264/2 264/88 350/150 350/267 walls oflight valves. This is especially necessary when [Sl] it. Cl. H018 4/00.large sized valves Conain a liquid Suspension To re Fleld of Search 592,duce hi distortion of Shape and thereby eliminate 350/147 bowing, anamount of fluid is withdrawn from the cell 881 94; 161/45 52/573 65/58prior to sealing the cell hermetically. Fluid is withdrawn, therebyreducing the pressure on the inside of [56] References and the walls,until the pressure at least one point inside the U ITE STATES PATENTSwalls, and preferably toward the lower center of a light 1,370,9743/1921 Kirlin 161/45 valve positioned vertically, is substantially equalto the 2,756,467 7/1956 Etling atmospheric pressure on the outside ofthe light valve 3,470,049 9/1969 Reusch 161/45 X walls, 3,535,098lO/l970 Babc'ock 65/58 X 3,592,526 7/1971 Dreyer 350/159 25 Claims, 4Drawing Figures PAIENIED JUL 1 0 I973 FIG. 2

INVENTORS F0 R L N In: F vi! MATTHEW I FRANCIS c. LOWELL BY ATTORNEYBACKGROUND OF THE INVENTION This invention refers to bowing of thinplates (sometimes referred to herein as walls) and especially to bowingof the thin, transparent glass plates used as the walls in light valves(cells) of the type in which a liquid suspension of particles isconfined in a space between two plates of glass. In light valves of thistype, electrically conductive, transparent coatings are applied eitherto the inside or outside surface of the plates so that an electricalpotential can be applied across the suspension in the space between theplates in order to activate and operate the light valve. In manyapplications, for example, when the valve is used as a window in abuilding, or as a shutter in a camera, the cell is used in a verticalposition. If the vertical dimension of the cell walls is small, forexample, 1 inch, as it might be in a camera shutter, the pressure of theliquid in the interior of the cell, which includes the hydrostaticpressure of the liq uid does not distort the shape of the plates to anappreciable, discernable extent because the hydrostatic head is small.However, when the vertical dimension of the cell is large, for example,12 inches or larger, the hydrostatic pressure of the liquid suspension(the pressure due to the weight of the column of liquid) causes apressure difference to exist between the inside and outside of a cellcausing a net outward force on the plates which bows and distorts theplates into a curved surface. This happens to a certain extent also inany cell in which the vertical dimension of the cell is large comparedwith the thickness of the plates. It should be appreciated that althoughone usually speaks of hydrostatic pressure in relation to the pressurecaused by the weight of a column of water, the term as used hereinrefers to the pressure caused by the weight of any fluid, for example,an organic liquid. The effect is pronounced, for example, in a verticalcell having two /4 inch or less thick glass plates, and whose length andwidth dimensions are, for example, 2 feet by 2 feet, and having a liquidlayer about mils thick between the plates. The aforesaid bowing ordistortion of the plates will lead to a light valve that is not uniformin suspension thickness. The plates will be separated more at one pointthan at another point. Therefore, the thickness of the suspensionthrough which the light passes will be different in different parts ofthe cell. This will result in non-uniform optical density from point topoint in the cell. For example, in a window, this produces a darker areain the center or below the center of the cell, and a lighter area aroundthe edges of the cell or at the top, of the cell. The bowing also hasthe undesirable result, since it creates a curved surface, of producinga lens effect to further distort transmission of light through the lightvalve. Another undesirable effect of the bowing is that the non-uniformseparation of the bowed electrodes produces non-uniform electric fieldsin different areas of the cell. The light valve opens more where theseparation is small because the field is larger there. The valve opensless where the separation is large because the field is smaller. Thisadds to the darkness of the cell in the bulged area, and adds to therelative lightness along the edges or at the top of the cell. It is thusan object of this invention to eliminate or reduce this bowing andresulting distortion in cells and devices having walls not strong enoughor too thin to prevent bowing, and especially in the walls of lightvalves.

SUMMARY OF THE INVENTION A method is disclosed for reducing bulging(bowing) caused by fluid pressure which occurs in thin, glass cells,such as light valves, especially when they are used in a verticalposition with a liquid substance between the cell walls. To eliminatethe bowing, an amount of fluid is withdrawn from between the cell walls(i.e., from inside the light valve) under conditions whereby theatmosphere surrounding the cell cannot enter the cell and then the cellis sealed hermetically. This re duces the pressure inside the cell sothat at at least one point (usually toward the lower center of a cell)the in ternal cell pressure is preferably approximately equal to thecorresponding pressure outside the valve. Alternatively, apredetermined, correct amount of fluid is placed inside the cell and byeither the aforesaid suction or squeezing methods, the fluid is causedto occupy substantially all of the volume between the light valve wallswithout causing bowing to occur.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a perspective view of alight valve.

FIG. 2 is a cross-sectional view of the light valve of FIG. 1 before ithas been filled with fluid suspension.

FIG. 3 is a cross-sectional view of the light valve of FIG. ll after ithas been filled with fluid and before the pressure has been relieved bythe method of this invention.

FIG. 4 is a cross-sectional view of the light valve of FIG. 1 after thepressure has been relieved by the method of this invention.

PREFERRED EMBODIMENT OF THE INVENTION In FIGS. I and 2 are shown atypical light valve involving this invention. The light valve consistsof two transparent plates 2 and 3 which are preferably constructed ofplastic, glass, or other strong transparent material. These plates arepositioned so they are substantially parallel and are then sealedtogether by a'suitable sealant 4 which is placed around the entireperiphery of the light valve to form an enclosed cell (1) inside thevalve. A small tube 6 is inserted at one end of the valve preferably theupper end (in FIG. 1). This tube is inserted so it creates a freepassage between the outer atmosphere and cell ll. The entire cell iscompletely sealed except for passage 7 through tube 6.

The liquid suspension is then poured into the cell through tube 6 sothat it completely fills the entire cell. The suspension containsparticles which change their orientation when an electric or magneticfield is applied to the cell. This substance will be described in moredetail hereinafter. The filling can be done when the cell is in avertical position with plates 2 and 3, vertical as shown in FIG. 2, orit can be done with the cell in a horizontal position. The suspensionitself, which is described in more detail in US. Pat. application Ser.No. 25,541 entitled Light Valve with Flowing Fluid Suspension andassigned to the assignee of the present invention is one that containsparticles of a material such as herapathite, in a carrier such asisopentyl acetate. The particles of herapathite will change theirorientation on the application of an electric or magnetic field. Toapply such a field to the suspension, the walls of the cell are coatedwith a transparent, electrically conductive material and then thesecoatings are connected by appropriate circuitry, as shown in FIG. 1, toa source of voltage. The suspension which normally (i.e., in the absenceof such field) appears dark (almost opaque) so as to let very littlelight radiation pass therethrough, becomes almost transparent, when avoltage is applied. It becomes transparent because the particles insuspension, the herapathite particles, become oriented on theapplication of the field so that they now block very little lightpassing through the cell. When the voltage source is removed, Brownianmovement causes the particles to become disoriented once again and thesuspension again becomes dark.

As aforementioned, one of the problems with these cells is that whenplates 2 and 3 are vertically oriented and are more than a few inches inheight, and especially when they are a few feet in height, the plateswill tend to bow and distort the proper functioning of the light valve.This is more clearly shown in FIG. 3. It will be appreciated that inFIG. 3 the bowing and usual relative thickness of the layer of fluidsuspension have been greatly enlarged to accentuate the effect forpurposes of illustration. This bowing will occur when the panel isfilled with fluid suspension; the bowing is caused by the pressure ofthe fluid in the cell being greater than the pressure outside the cell.The difference is approximately equal to the pressure caused by theweight of the column of liquid in the cell. Stated another way, theliquid in such a cell is at the equivalent of atmospheric pressure plusits hydrostatic pressure, whereas, outside the cell only atmosphericpressure exerts a force on the cell walls. The actual bowing effect isaggravated by the fact that the cell walls can bend more near theircenters than at their edges because the edges are cemented to eachother. This hydrostatic pressure of the fluid increases from the top tothe bottom of the cell due to the weight of the fluid in the cell. Thebowing will also be most pronounced slightly below the midpoint of thecell because of the weight of the fluid (hydrostatic pressure). Thebowing can lead, as aforementioned, to uneven light transmission effectsof the cell due to uneven distribution of fluid suspension and to lenseffects.

Also, this non-uniform thickness of the fluid suspension will cause avariation in field gradient as well, which can cause a pronouncedvariation in transmission response from point to point when the cell isactivated. Also, bowing and distortion of the glass walls can causefracturing of the walls of the cell, rupturing of the seals and otherserious damage. Thus, it is of the utmost importance that the cell wallsbe returned to, or kept in approximately the same flat, parallelrelationship as before the fluid was added to the cell so that the cellis able to operate properly. It will be appreciated, of course, thatthis invention can also be applied to light valves where one wishes tomaintain any predetermined spacing even a tapered or other non-constantspacing between the cell walls.

The term fluid" shall include but not be limited to fluid suspensions.The term liquid shall include but not be limited to liquid suspensions.

It should also be noted that although only one opening is shown in theaccompanying drawing through which fluid may pass, it is often desirableto have a second opening (not shown in the drawings) usually at or nearthe top of the light valve to allow gas such as air to escape from thelight valve while liquid suspension, for example, is placed in the cell.Such second opening,

if used, should preferably be hermetically sealed before performing thesuction or squeezing methods and final hermetic seal herein described.

The method of this invention for solving bowing will now be described indetail.

Utilizing tube 6, the cell is completely filled with liquid suspension.In filling the cell it is preferable that there be no air bubble orbubbles remaining in the liquid suspension, or cell or in tube 6.However, if one or a few small bubbles, or a small air space remains itis tolerable, provided that the volume of these bubbles or of the airspace is small compared to the total volume of the cell. The cell, beingnow completely filled with liquid suspension, will begin to bulge andwill assume the shape as shown in FIG. 3 with the plates 2 and 3 beingbowed and distorted outwards because of pressure differential caused bythe hydrostatic pressure of the liquid suspension. The next step in themethod of this invention is to apply a controlled vacuum to the open end10 of tube 6 to reduce the pressure of the liquid in the tube and in theinterior of the cell. A suction device to apply this vacuum, (as will bedescribed hereinafter) can be a suction pump or other similar device.The suction device is connected in an air tight manner. For example, thesuction device can contain a tube which is forced into tube 6. Thesuction device is then operated and liquid is removed. The pressure inthe cell will thereby be reduced and plates 2 and 3 will begin to returnto their original flat shape and the bowing will begin to be relieved.Fluid is removed until the walls assume a flat, parallel shape andrelationship to one another, as shown in FIG. 4. In other words, enoughliquid in the cell will be removed under suction conditions until thepressure of the liquid at the lower center of the cell 11 (place ofmaximum bulge) is reduced to approximately the atmospheric pressure onthe outside 12 of the cell on the opposite side of the cell wall.

The hydrostatic pressure in the liquid increases linearly in thedownward direction from the upper level of the liquid and isapproximately constant in any one horizontal plane. The bulging however,does not increase linearly from the top of the panel to the bottom.Bulging is not constant in any one horizontal plane, because the sealantaround the edges of the cell binds the two glass plates to each otherand constrains the plates from bulging at the edges. The platestherefore bulge into a shape that is determined by the shape and size ofthe cell, the thickness of each of the glass plates, the elasticconstants of the plates, their specific constraints, and the specificgravity of the liquid suspension. The plates bulge into a generallyspherical shape with the crown of the bulge being generally below thegeometric center of the cell.

Further detail will now be presented to explain some of many examples ofsuction devices that can be used with this invention, and how thesedevices are used. A hypodermic syringe, as shown in FIG. 3, can beattached or fastened tightly to the open end [0 of the tube 6, as byinserting it into the tube so that no air can enter the cell. Theplunger of the syringe is depressed before being attached to the openend of the tube. The plunger is then slowly withdrawn to withdraw fluidinto it until the proper pressure is obtained inside the cell and theplates 2 and 3 are substantially flat. Thiscan be done by visualobservation or various gauges. After the cell walls are flat, the end ofthe tube is then sealed off as shown in FIG. 4 at point 13 and then thehypodermic syringe or other suction device is removed. A sturdy rubberbulb can be used in place of the hypodermic syringe by first compressingthe bulb by hand and then fastening the open end of the bulb to the opentube '6. The bulb is then allowed to expand under its own elasticity byslowly releasing the operators hand so that liquid is drawn into thebulb until the pressure inside the cell is adjusted so that the plates 2and 3 are flat. When they are flat, the tube 6 is sealed off as at 13 inFIG. A and the bulb is removed. As aforementioned, any other devicewhich is a source of reduced pressure or vacuum or partial vacuum can beattached to the open end of the tube 6 to accomplish the same purpose.

One convenient method for sealing off the end of the tube 6 is to makethe tube out of glass and seal it off using a conventional method knownin the art, of glass blowing, that is, by heating it and then pinchingthe glass tube so that it seals itself. Another method is to use a softmetal tube, for example, copper or soft brass, and to seal the tube bysqueezing it closed with an appropriate tool, such as a crimping tool ora pair of pliers. The tube can be bent almost to 180 and then sealed atthe bend by pressure. Also, suitable adhesives can be used to seal thetube. The important consideration is that the tube be hermeticallysealed against fluid entering or leaving enclosure 1.

When liquid is drawn (sucked) out of the top of the cell as taught inthis invention, the atmospheric pressure equivalent of the liquidremaining inside the cell is reduced to a fraction of what it was. Thehydrostatic pressure of the liquid in the cell is of course unchanged,but now the total pressure inside the cell, the hydrostatic pressureplus the reduced atmospheric pressure equivalent of the liquid isreduced. It will be appreciated that the pressure in the cell is notconstant throughout since the hydrostatic pressure increases downwardlyin the cell. However, the point of major interest is the point ofmaximum bulge, which as aforementioned, will occur below the center ofthe cell. Thus the atmospheric pressure is reduced until the cell isflat at this point of maximum bulge.

However, the pressure on the inside of the plates above the midpoint isless than the outside atmospheric pressure, and the inside pressure onthe plates below the midpoint is greater than the outside atmosphericpressure. The plates above the midpoint tend to bulge inwardly. However,in practice it is found that this inward bulging of the upper portion ofthe plates, is negligible for most applications, e.g., windows ofmoderate size and television contrast enhancing filters. The in ventiondisclosed herein suceeds in reducing all bulges to the point where theyare not noticeable in most applications.

The foregoing analysis enables us to calculate the approximate reductionin pressure, AP, that must be made by the suction or vacuum device thatis used in this invention, assuming, for simplicity, that the pressurein the cell when vertical at half of its maximum height equals standardatmospheric pressure (760 mm of mercury at sea level). The calculationis made by the formula: AP= /gHD where AP is the reduction in pressurein dynes per square centimeter per second per second; H is the verticaldistance in centimeters, between the upper level of the liquid to thebottom of the liquid in the cell; and D is the density of the liquidsuspension in grams per cubic centimeter,

For example, if a rectangular cell is held with two of its edgesvertical, and the vertical dimension of the inside of the cell iscentimeters; and the density of the liquid suspension is 1.5 grams percubic centimeter, and the acceleration of gravity is 980 centimeters persecond per second; then the drop in pressure AP, that should be producedby the suction device at the top of the panel is 73,500 dynes per squarecentimeter, which is approximately 0.073 atmosphere of pressure.

Alternatively, instead of withdrawing fluid from the cell, the walls ofthe cell can be squeezed together to force air out of the cell underconditions where no air can enter the cell to cause a reduction in thepressure in the cell.

Briefly, restating the operation once again, and amplifying thereon, thelight valve is filled with liquid suspension, either in the horizontalor vertical position, and its walls are either squeezed together toeliminate the bowing or vacuum applied to accomplish the same objective.By either method, air is preferably all forced out of the cell togetherin some instances with some liquid, and the cell is hermetically sealed,with liquid filling all or substantially all of its inner volume. If thewalls of the cell are to be squeezed together, a mechanical guidefixture or similar squeezing apparatus can be employed. Whether one usessuction or squeezing, and whether one fills the cell in the horizontalor vertical position, this invention will reduce the bulging problem. Itwill also be appreciated that when the exact capacity of the cell isknown, the exact, correct amount of fluid can be placed in the cell. Theaforesaid exact amount of fluid is the amount that would be left in thecell after the bulging is eliminated as previously described. Of course,before the cell is corrected to eliminate bulging this amount of liquidwill not completely fill the cell; the remainder of the panel willprobably be filled with gas or air. Bulging will of course take placefor the reasons previously cited. The same methods as previouslydiscussed will be used to fill the cell and correct for bulging, e.g.,either squeezing or withdrawing by suction. However, in both cases onlythe gas or air will be withdrawn; no liquid will be removed. The liquidwill simply redistribute itself through the inside of the panel once theair is removed. Now, as before, the pressure of the liquid at the pointwhere the maximum bulge was, will approximately equal the atmosphericpressure outside the panel. The bulging will then be eliminated. Thisprocedure simplifies filling of large numbers of cells as in massproduction.

The significant feature of this invention is that regardless of thedistortion due to pressure, the walls are returned to almost theiroriginal shape.

It wll be appreciated thus, that a highly efficient way of reducing thebowing in light valves is provided.

While specific embodiments of this invention have been illustrated, itwill be appreciated that the invention is not limited thereto, sincemany modifications may be made by one skilled in the art which fallwithin the true spirit and scope of the invention.

We claim:

1. The method of causing at least one wall of a deformed cell to returnsubstantially to its undeformed condition by reducing the pressure offluid inside the cell where the pressure of the fluid is at least at onepoint inside the wall of the cell greater than pressure of fluid at acorresponding point on the other side of the wall outside the cellcomprising the steps of:

causing the wall to return to its undeformed condition by reducing thepressure inside the cell by re moving fluid until the pressure of thefluid inside the cell at that point is substantially equal to thepressure of the fluid outside the cell at the corresponding point, and

hermetically sealing the cell.

2. The method of claim 1 wherein prior to reducing the pressure insidethe cell by removing fluid the pressure at the point inside the cell iscaused by the sum of hydrostatic pressure and a pressure equal toatmospheric pressure and the pressure at the corresponding point outsidethe cell is caused by atmospheric pressure.

3. The method of claim 2 wherein that part of the pressure inside thecell caused by the pressure equal to atmospheric pressure is reduced byremoving fluid so that the part of the pressure equal to atmosphericpressure plus the hydrostatic pressure at the point inside the cell aresubstantially equal to the atmospheric pressure outside the cell.

4. The method of claim 3 wherein the fluid in the cell comprises aliquid.

5. The method of claim 4 wherein the fluid removed is some of theliquid.

6. The method of claim 3 wherein the fluid comprises a liquid and a gas.

7. The method of claim 6 wherein the fluid removed comprises the gas.

8. The method of claim 6 wherein the fluid removed comprises the gas andsome liquid.

9. The method of claim 8 wherein the fluid is withdrawn under suctionconditions.

10. The method of claim 8 wherein the fluid is removed by reducing thevolume of the cell without allowing any other fluid to enter the cell.

11. The method of claim 9 including means defining the cell comprisingtwo interconnected substantially parallel, flat walls at least one ofwhich is transparent.

12. The method of claim 11 wherein the cell comprises a light valve.

13. The method of claim 12 wherein the liquid inside the cell comprisesa liquid suspension having particles in suspension which can have theirorientation changed by the application of an electric or magnetic fieldto control the passage of radiation through the cell.

14. The method of claim 13 wherein the liquid at the point in the cellat which its pressure is greater than the pressure of the liquid outsidethe cell causes the walls of the cell to bulge outwardly, and whereinthe step of reducing the pressure causes the walls to return to theirflat, parallel form.

15. The method of claim 14 wherein the fluid outside the cell comprisesair at atmospheric pressure.

16. The method of claim 15 wherein a tube is mounted to connect with theinterior of the cell and whereby the fluid is removed through said tube.

17. The method of claim 16 wherein the step of removing fluid includesapplying a suction device to the tube so as to prevent fluid such as airfrom entering the cell while the fluid inside the cell is being removed.

18. The method of claim 17 wherein hermetically sealing the cellincludes the step of collapsing the tube so that it is hermeticallysealed.

19. The method of claim 18 wherein the step of reducing the pressure iseffected when the cell is in a substantially vertical orientation withthe walls of the cell vertically oriented.

20. The method of claim 19 wherein the suction device comprises asyringe.

21. The method of claim 10 including means defining the cell comprisingtwo interconnected substantially flat, parallel walls.

22. The method of claim 21 wherein the fluid is removed by moving thewalls toward each other and permitting some fluid to be ejected througha means comprising an opening.

23. The method of claim 22 wherein the walls of the cell are squeezedtogether to cause the ejection offluid.

24. The method of filling a cell where one wall is deformable so thatthe cell will not become deformed when fluid is placed thereincomprising the steps of:

determining substantially the maximum amount of fluid which when placedin the cell will enable the cell to remain in substantially its originalundeformed shape, where if more than this amount of fluid were placed inthe cell the walls of the cell would deform, the determination beingmade by determining the amount of fluid that will remain in a cell whichhad deformed walls where fluid was removed to return the walls of thecell to their original undeformed condition where the deformation iscaused by the pressure of the fluid inside the wall at a point beinggreater than the pressure of fluid at a corresponding point on theotherside of the wall outside of the cell,

filling the cell with this amount of fluid, and

hermetically sealing the cell.

25. The method of claim 24 wherein as the cell is being filled with theliquid the cell is positioned in a substantially horizontal direction.

(SEAL) A s Atteste. j g

MCCOY M. GI BSQN JR V MARSHAL-1:5 DAEfi- I aAttesFing q isionj r-Plfents.

' L. UNITED STATES. "PATEr- OFFICE I CERTIFICATE OF CO"RRECTION3,744,126 Da July in, 1973 Patent No.

Inventor) -I-1a'tthew F orli m'. and Francis C. Lowell is .cerufied thatppears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

In the abstract:

Line 8, between "at" and "least" -at should be inserted.

signed and sealed this 7th y of a u r

1. The method of causing at least one wall of a deformed cell to returnsubstantially to its undeformed condition by reducing the pressure offluid inside the cell where the pressure of the fluid is at least at onepoint inside the wall of the cell greater than pressure of fluid at acorresponding point on the other side of the wall outside the cellcomprising the steps of: causing the wall to return to its undeformedcondition by reducing the pressure inside the cell by removing fluiduntil the pressure of the fluid inside the cell at that point issubstantially equal to the pressure of the fluid outside the cell at thecorresponding point, and hermetically sealing the cell.
 2. The method ofclaim 1 wherein prior to reducing the pressure inside the cell byremoving fluid the pressure at the point inside the cell is caused bythe sum of hydrostatic pressure and a pressure equal to atmosphericpressure and the pressure at the corresponding point outside the cell iscaused by atmospheric pressure.
 3. The method of claim 2 wherein thatpart of the pressure inside the cell caused by the pressure equal toatmospheric pressure is reduced by removing fluid so that the part ofthe pressure equal to atmospheric pressure plus the hydrostatic pressureat the point inside the cell are substantially equal to the atmosphericpressure outside the cell.
 4. The method of claim 3 wherein the fluid inthe cell comprises a liquid.
 5. The method of claim 4 wherein the fluidRemoved is some of the liquid.
 6. The method of claim 3 wherein thefluid comprises a liquid and a gas.
 7. The method of claim 6 wherein thefluid removed comprises the gas.
 8. The method of claim 6 wherein thefluid removed comprises the gas and some liquid.
 9. The method of claim8 wherein the fluid is withdrawn under suction conditions.
 10. Themethod of claim 8 wherein the fluid is removed by reducing the volume ofthe cell without allowing any other fluid to enter the cell.
 11. Themethod of claim 9 including means defining the cell comprising twointerconnected substantially parallel, flat walls at least one of whichis transparent.
 12. The method of claim 11 wherein the cell comprises alight valve.
 13. The method of claim 12 wherein the liquid inside thecell comprises a liquid suspension having particles in suspension whichcan have their orientation changed by the application of an electric ormagnetic field to control the passage of radiation through the cell. 14.The method of claim 13 wherein the liquid at the point in the cell atwhich its pressure is greater than the pressure of the liquid outsidethe cell causes the walls of the cell to bulge outwardly, and whereinthe step of reducing the pressure causes the walls to return to theirflat, parallel form.
 15. The method of claim 14 wherein the fluidoutside the cell comprises air at atmospheric pressure.
 16. The methodof claim 15 wherein a tube is mounted to connect with the interior ofthe cell and whereby the fluid is removed through said tube.
 17. Themethod of claim 16 wherein the step of removing fluid includes applyinga suction device to the tube so as to prevent fluid such as air fromentering the cell while the fluid inside the cell is being removed. 18.The method of claim 17 wherein hermetically sealing the cell includesthe step of collapsing the tube so that it is hermetically sealed. 19.The method of claim 18 wherein the step of reducing the pressure iseffected when the cell is in a substantially vertical orientation withthe walls of the cell vertically oriented.
 20. The method of claim 19wherein the suction device comprises a syringe.
 21. The method of claim10 including means defining the cell comprising two interconnectedsubstantially flat, parallel walls.
 22. The method of claim 21 whereinthe fluid is removed by moving the walls toward each other andpermitting some fluid to be ejected through a means comprising anopening.
 23. The method of claim 22 wherein the walls of the cell aresqueezed together to cause the ejection of fluid.
 24. The method offilling a cell where one wall is deformable so that the cell will notbecome deformed when fluid is placed therein comprising the steps of:determining substantially the maximum amount of fluid which when placedin the cell will enable the cell to remain in substantially its originalundeformed shape, where if more than this amount of fluid were placed inthe cell the walls of the cell would deform, the determination beingmade by determining the amount of fluid that will remain in a cell whichhad deformed walls where fluid was removed to return the walls of thecell to their original undeformed condition where the deformation iscaused by the pressure of the fluid inside the wall at a point beinggreater than the pressure of fluid at a corresponding point on theotherside of the wall outside of the cell, filling the cell with thisamount of fluid, and hermetically sealing the cell.
 25. The method ofclaim 24 wherein as the cell is being filled with the liquid the cell ispositioned in a substantially horizontal direction.